1. Field of the Invention
The invention relates to radiofrequency identification devices used to tag or identify objects, and in particular to radiofrequency identification devices that may be transiently programmed, remotely self-calibrated for synchronization, remotely powered during data communication, and implemented using a high frequency isolated monolithic rectifier.
2. Description of the Prior Art
Low Power Antifuse RFID Memory
A radiofrequency identification device or tag (hereinafter referred to throughout this specification as an "RFID tag") is a device which can be attached or associated with another object, which the RFID tag is then used to identify when queried remotely by an interrogating circuit. The RFID tag is thus preprogrammed, or programmable after association with the object to return a signal to the interrogating circuit to provide selected information concerning the attached object which is within the zone of the interrogating circuit. More simply, a small electronic tag is attached to an object and the tag is read by a reader. New data can be added to the tag by a programmer, and the data received from the tag can either be read from a display screen, stored or later down-loaded to a personal computer, or linked directly to a computer system.
RFID tags have an advantage over bar codes and bar code readers which perform similar functions in that the RFID tag may be imbedded within the object and still be read as long as the interlying material between the RFID tag and the reader is not conductive. Therefore, line of sight is not required for an RFID tag, which is required in any type of bar code reader. This allows the RFID tag to function in very difficult environments. Further, the RFID tag has the capacity for having digital data being added after it is attached to the object, has a greater data capacity and can be read at distances far greater than those achievable through optical bar code readers.
To date, the cost of RFID tags, however, has limited the market penetration of the device because of the high cost associated with such RFID tags.
Ishihara et al., "Antifuse Memory Device with Switched Capacitor Setting Method," U.S. Pat. No. 5,299,152 (1994) describes a capacitively charged pump circuit in which signals are applied to gates to charge pump a capacitive element for the purposes of subjecting an antifuse coupled to the capacitor to dielectric breakdown for programming purposes. The charge pump is certainly not transient and required considerable power, making both the circuitry and methodology impractical for most RFID tags.
Therefore, what is needed is an RFID tag which can be manufactured at low cost, programmed and run at low energies, programmed in the field, and still retain each of the advantages of RFID tags over the bar codes as discussed above.
Self-Calibration of Timing for an RFID Tag
A persistent problem with low power RFID tags is the remote calibration of the circuit to allow information sent to the RFID tag to be decoded accurately. If the clock signals or discrimination levels are not properly calibrated on the RFID tag properly, or if such calibration is not maintained as environmental circumstances of the tag changes, then transfer of information to and from the tag becomes unreliable. First consider some prior art self-calibration schemes used in other applications.
Goffin, II, "Method and Apparatus for a Calibrated Electronic Timing Circuit," U.S. Pat. No. 5,117,756 (1992), adjusts an internal oscillator by comparing its output to a control signal. A precision calibration pulse is applied to the timing circuit which counts the output cycles of a variable frequency oscillator during the period of the pulse. This count is stored and compared to the reference count to produce an error count. The error count is combined with a previously stored control signal to produce a new control signal that drives the output of the oscillator to a new frequency.
Goffin's calibration circuit is used for a calibrated time delay circuit for delayed ignition of explosive products. The application is to minimize the effects of rock blasting on nearby structures by reducing peak-to-peak amplitude of frequency of ground vibration produced by the blast by timing the ignition of the plurality of explosive charges. Goffin achieves this with an onboard calibration pulse derived from a time reference and then calibrates the detonation timing circuitry to it in order to compensate for fluctuations, ambient temperature, humidity and pressure that may cause a variation in the local oscillator rate. Goffin is not concerned with a remote communication circuit, but rather with calibration of a plurality of detonator circuits with each other, all connected by hard wiring.
Weaver, "Electronic Frequency Control for Radio Receivers," U.S. Pat. No. 2,501,883 (1950), also generally describes a circuit which adjusts an internal oscillator by comparing its output to a control signal. Weaver's object is to provide a local beat frequency and proper frequency relationship to a carrier. In particular, Weaver seeks to keep a local oscillator adjusted to generate a wave having substantially the same frequency relationship to the received carrier as existed before a carrier fade commenced. Weaver achieves this by combining a wave derived from the received carrier with a locally generated reference wave to produce a controlling voltage whose magnitude and polarity are determined by the vector sum or difference of the combined waves. A reactance tube is connected to control the frequency generated by the oscillator. The grid of the reactance tube is coupled to two biasing circuits, which are in turn driven by the controlling voltage.
Information transmitted to an RFID tag is decoded either by detecting the variation in the amplitude (AM) or frequency or equivalently the phase shift or time delay (FM) of the carrier signal, depending upon the communication protocol which has been chosen as the standard. Changes in the amplitude of the carrier signal are economically detected by conventional RFID tag receivers, but are susceptible to noise interference. In the transmission of digital information, the loss of just a single bit of data can, if inappropriate, cause catastrophic consequences.
Detection of a change of frequency is less susceptible to noise interference but requires that the RFID tag receiver be capable of detecting changes in the carrier signal as against a calibrated standard. RFID tag receivers typically rely on some type of internally tuned circuit to compare the incoming signal to the standard in order to detect the frequency variation. However, if the RFID tag receiver is depended upon the incoming carrier signal as a reference itself, as is almost always the case with RFID tags, it is inherently impossible to detect changes in the incoming signal using standard FM techniques.
Therefore, what is needed is a method whereby a remote RFID tag circuit can calibrate itself with respect to an input signal allowing for information to be decoded after the completion of the calibration, which decoding would not have been possible before calibration.
Bit and Frame Synchronization
It is further well known that in every communication protocol some means is required to determine which bit is the first data bit in a digital data transmission. This determination process becomes more difficult in wireless devices. Since the transmitting device may be at the extreme end of an operating range, noise and signal dropout make reliable detection of digital data difficult, as well as validation of synchronization at both the bit and data frame levels.
Wireless devices typically move into and out of range quickly. Therefore, it is an advantage if the bit synchronization is achieved in a minimum of time so that the wireless device can start looking for the frame synchronization as soon as possible. Typically, the ability of the wireless device to do this makes a critical difference in whether the communication is successful or not.
In some RFID tags a violation of the "normal" data protocol is used to identify the beginning of a data frame. Milheiser, U.S. Pat. No. 4,730,188 teaches that using Manchester encoding defines a protocol that has a change of state every bit time. The data frame includes a frame marker which contains a preamble with a specific bit pattern followed by a violation of the Manchester bit timing protocol in which the rate of change of state is decreased to 11/2 bit times for 3 bit intervals, followed by the identification of the data in which the bit rate is restored. This type of bit synchronization is usable in Milheiser's application where the tag transmits to a reader, but the RFID tag does not receive coded data.
Even in Milheiser's application, some disadvantages exist. Since Milheiser must first achieve bit synchronization, the use of phase lock loop is necessary, which uses feedback of a received frequency transition to adjust a rate to become synchronized. If the protocol violation occurs at that moment in time, the phase lock loop will attempt to synchronize to a rate 1/3 lower. The result is that the bit synchronization is delayed in a frame which could have been read but is not.
The prior art has also devised an alternative approach which is not subject to the disadvantages of the Milheiser protocol. According to this alternative approach, a unique data value is assigned as a frame marker, which unique value is then prohibited from being used as a data pattern. This approach allows for all or any part of the data frame to be used for a bit synchronization. The unique data pattern must not be a pattern which the user would ever want to use in the data since it is prohibited to prevent ambiguity. If the unique data pattern is required, an alias is created which is then translated back to the prohibited value at a later time. This alias creation and retranslation is an awkward solution in most applications.
This approach also leads the user to choose a value such as all 1's or all 0's for the unique pattern. Such a pattern is not a practical choice since the memory in most application devices is normally initialized to either all 1's or 0's in any case.
Lee, "Fault and Error Detection Arrangement," U.S. Pat. No. 4,429,391 (1984), describes a fault and error detection arrangement for detecting transmitting and routing errors made in a central data transmitter and receiver communicating with peripheral circuits in which parity bits of certain data words are transmitted by the central data transmitter after being intentionally inverted by central parity inverter in a known sequence. The purpose of the inversion every predetermined number of frame is used for synchronization. In particular, the central parity inverter inverts the parity bit every ninth data word in response to parity control signals transmitted by the sequence generator.
Thus, Lee looks for repetitive parity violation on a periodic basis in order to establish timing. The drawback, however, is in cases where there is signal fade which is common with RFID tag devices, an insufficient number of synchronization parity violations may have been received in order to reliably establish the pattern, or that the pattern may be unreliably transmitted thereby leading to substantial errors in synchronization.
Terrab et al., "Method and Apparatus for Ensuring CRC Error Generation by a Data Communication Station Experiencing Transmitter Exceptions," U.S. Pat. No. 5,195,093 (1993), shows a unique code generation scheme using parity change. Each byte of serial bit stream is sequentially transmitted. If a transmitter exception occurs, the byte before the exception is transmitted normally. However, only the first seven bits of the last byte are transmitted. The parity bit is sent as an eighth bit of the last byte ensuring odd parity for the previous bit stream. Thereafter, a byte even parity is sent to ensure the overall message has odd parity. A receiving station interprets two consecutive bytes having the predetermined data pattern as the CRC, thus assuring that the receiving station will reject the frame.
Therefore, what is needed is some type of method for achieving bit and frame synchronization in digital signals transmitted between radiofrequency identification tags and readers or writers in a way which is very simple and yet efficient to implement in an integrated circuit.
Data Communication and Power
Typically RFID systems transmit a carrier signal and then divide down the carrier frequency on the tag to use the signal as an internal clock. The information stored on the tag is then sequentially transmitted from the tag. A tag which operates in this manner is a read-only tag. The information in the tag must be entered during the manufacturing of the tag by making direct electrical contact to external connectors, or by having a battery or charged capacitor physically connected to the tag.
There is a recognized need to be able to add information into the RFID tag remotely in the field rather than having information loaded into the tag only during its manufacture. Remote programming or wireless programming without any physical contact to the tag being made can only be accomplished if power and information are both supplied simultaneously to the tag. Programming the tag requires substantially more power than simply reading the tag. Prior art methods for remote programming rely on AM modulating the signal or FM modulating the frequency to communicate with the tag. Again AM is susceptible to interference through noise and FM requires significant sensing or detection circuitry to be built within the tag.
Consider first how the prior art has transmitted data and power on carrier signals. Kobayashi et al., "Digitally Remote Control Transmission Apparatus," U.S. Pat. No. 4,914,428 (1990), describes the use of a time duration alteration between synchronization signals. The transmitted coded digital instruction is composed of a sequence of synchronization pulses having a predetermined period and data pulses which are inserted between the successive synchronization pulses at predetermined positions dependent upon whether data pulses represent a 0 or 1 bit. The receiving circuit distinguishes between the 0 and 1 bits by detecting the length of the interval between the leading edge of a synchronization pulse and the leading edge of an adjacent data pulse and determines the existence of noise, if more than one data pulse is detected between successive synchronization pulses. The length of each data word which is sent is constant regardless of the numbers of 1's and 0's in the word so that the detection of more than one data pulse between successive synchronization pulses of a constant period is interpreted as being noise. Kobayashi thus uses pulse delay in order to distinguish between binary 0's and 1's from a periodic timing pulse.
Stobbe et al., "Portable Field-Programmable Detection Microchip," U.S. Pat. No. 5,218,343 (1993), shows a system for transmitting both power and information to a remote circuit using external charging capacitors, an internal oscillator, an AM signal from the chip, and a period variation to distinguish between binary 0's and 1's. The RFID tag chip in Stobbe is provided with a charging capacitor capable of storing electrical energy from the RF signal so that the microchip in the tag can be powered during pulse pauses of the RF signal. The microchip includes a memory circuit for storing the identification code of the microchip in a code generator that is coupled to the memory circuit for generating an RF signal that is modulated with the identification data. A switching element detunes a resonant circuit in the microchip when the identification data is transmitted back to the read/write device. The resonant circuit serves to field program the memory circuit of the microchip and the tag by receiving pulse/pause modulation signals (PPM) of the RF carrier signal to allow the identification code of the tag to be altered. Stobbe describes a mixing of commands and data by AM modulation.
Kriofsky et al., "Inductively Coupled Transmitter-Responder Arrangement," U.S. Pat. No. 3,859,624 (1975), describes a conventional low frequency RFID tag which is powered through inductive coupling so that the tag generates uniquely coded information sent back to a reader. The coded information is not however transmitted as part of or modulation of the inductive power signal.
Therefore what is needed is a method for delivering energy remotely to an RFID tag while at the same time transmitting information to the tag. Some means must be found whereby the carrier signal broadcasts to the tag can both deliver data which is sensed and power at the same time.
Rectifiers for RFID Tags
In current designs for RFID tags, the carrier signals are rectified through the use of on-chip transistors which are typically slow, there being no need for fast response times. A conventional full wave bridge rectifier using 4 diodes as depicted in FIG. 9 is typically not used in an RFID tag because the parasitic junctions formed in a conventional monolithic integrated circuit, by which such a diode bridge would be made, cause the structure to be inoperable in the application of an RFID tag.
FIG. 10 is a cross section of a typical integrated circuit layout for a rectifier bridge as shown in FIG. 9. Junctions 86 in the circuitry of FIG. 10 become forward biased when applying an AC signal on contacts 88 and 90 corresponding to the same junctions referenced in the schematic of FIG. 9.
Trying to implement a transistor structure in a high frequency range such as 915 Megahertz or 2.5 Gigahertz in an RFID tag leads to even further difficulties. First, MOS transistors must be scaled in order to operate at such speeds or a high speed bipolar device such as using a biCMOS process. Both types of transistor technology require a much higher cost to manufacture and the parasitic capacitance in such devices have a substantially greater effect as the frequency of operation is increased.
Therefore, what is needed is a method of rectifying a carrier signal transmitted to an RFID tag for the purposes of powering the tag which is not subject to the defects of the prior art.